1. Field of the Invention
The present invention relates to a servo-type vibration detector.
2. Description of Related Art
A conventional servo-type vibration detector is an apparatus that detects vibration using a servo mechanism, in which a position of a pendulum is controlled. When the pendulum is displaced because of external vibration, a current is supplied to a coil wound around the pendulum, and thereby the pendulum is held at a predetermined position. The current supplied to the coil at the time detects vibration amounts, such as a speed, a rate of acceleration, and the like.
The servo-type vibration detector of this type is provided with a movable plate and a fixed plate, the movable plate being provided to a coil bobbin functioning as a pendulum supported by a support spring and housed in a main body case, the fixed plate being provided to an internal periphery of the main body case. The plates are provided opposite to each other. A rate of acceleration applied to the coil bobbin is detected, based on measurement of a change amount of electrostatic capacitance between the movable plate and the fixed plate. Further, the vibration detector described above is directly mounted to a measurement object, such as a seismic measurement device, a liquid crystal correction laser apparatus, a vibration isolator, and the like. Since the vibration detector is mostly mounted to a location difficult to directly access, such as in the earth or inside the apparatus, it can not be easily removed for inspection and maintenance. Thus, a test coil is wound around the coil bobbin, and then a predetermined test signal is applied to the test coil to vibrate the coil bobbin for testing. Thereby, it is confirmed whether the vibration detector operates normally (for example, Related Art 1). For the servo-type vibration detector as above, insulation is provided in order to avoid conduction between the plates (for example, Related Art 2).
[Related Art 1] Japanese Patent Laid-open Publication No. 2009-020057
[Related Art 2] Japanese Patent Laid-open Publication No. 2003-004522
When the vibration detectors disclosed in Related Arts above are applied with vibration at a rate of acceleration greater than a maximum rating of the vibration detectors, that is, vibration greater than an amplitude in which vibration can be detected, electrodes may collide with each other. Since the electrodes are insulated, an electrostatic capacitance between the electrodes is retained at a certain capacitance even during the collision. It is thus not easy to detect whether or not a contact is made, particularly when the contact is momentary. It is difficult to recognize whether or not a detection limit of the vibration detectors is exceeded. In addition, even when a rate of acceleration within a detectable range of the vibration detectors is applied to the pendulum in the above-described test with test vibration, the electrodes may collide in a case in which a structure of the vibration detectors is deformed due to aging degradation of the vibration detectors or a significant impact thereto. It is also difficult to easily determine a contact of the electrodes under the circumstance above, thus leading to an erroneous decision on whether the vibration detectors need to be replaced.